This invention relates to a method and apparatus for accurately detecting intracranial pressure and determining local cerebral blood flow by thermal and hydrogen clearance techniques and further relates to a cerebral probe for this purpose.
In the field of cerebrosurgery and neurosurgery, an important adjunct to the assessment and management of the acute head injury patient or the post operative patient is the measurement of intracranial pressure. The brain or cerebrum is protected in a container consisting of the skull and meninges. The container is filled with a fluid which is referred to as the cerebrospinal fluid (abbreviated CSF) and the cerebrum floats in the fluid. An organism membrane referred to as the dura is present inside the skull for maintaining air tightness within the skull. Various conditions such as a blow to the head causes such as an epidermal hemorrage, hydrocephalus and the like may cause an increase in internal pressure in the skull which is mainly attributable to an abnormal condition in the circulatory system of the cerebrospinal fluid. Elevated intracranial pressure is defined as an intracranial pressure (ICP) equal to or greater than 15 Torr using ventriculostomy, a subarachnoid bolt or a subdural catheter.
Internal pressure in the skull has been measured according to various methods. It has been general practice to measure intracranial pressure by epidural transducer methodology which involves drilling a hole in the skull cap and then inserting a tube or probe into a ventricle and breaking the dura to extract a pressure sample and measuring the sample by various pressure sensory techniques.
In more recent developments, pressure transducers of various types have been used for accurately measuring the internal pressure in the skull which has significant advantages in that the dura is not broken and the stress imposed upon the patient is significantly reduced. For example, see U.S. Pat. No. 4,354,504.
In addition to intracranial pressure, other parameters are of value to the physician treating traumatic or chronic cerebral conditions. One of these is measurement of regional cortical blood flow. Various techniques have been used in the prior art for determining blood flow in body organs particularly the brain including photoelectric plethysmography and light beating spectroscopy. One particularly effective technique is disclosed in U.S. Pat. No. 4,354,504 entitled "Thermal Diffusion Flow Probe". In this patent, a blood flow probe is used which utilizes two integrated circuit transducers each of which produces an output current proportional to the absolute temperature of the probe contact plate with which the temperature transducer is associated. One temperature transducer monitors the temperature of the cold plate of the probe and the other temperature transducer monitors the temperature of the hot plate. Each transducer output signal is transmitted from the probe so that the temperature of the hot plate and of the cold plate may be independently measured. Each of these signals is then input to a differential amplifier and a differential temperature signal is made available to a conventional monitoring device. A signal output by the temperature transducer monitoring the hot plate of the probe is also input to a comparator which level is compared with the reference signal. Should the hot temperature exceed the temperature associated with the reference signal, the comparator will cause the heat pump of the probe to be de-energized until such time as the hot plate temperature is reduced to a safe level. Additional features of the prior invention provide that both the hot and cold contact plates of the blood flow probe shall be at ground potential to eliminate hazardous shocks.
Another method of monitoring blood flow is the hydrogen clearance technique which was introduced in approximately 1965 by Knut Aukland, M.D. and others. Aukland discussed measuring local blood flow in an article entitled "Measurement of Local Blood Flow with Hydrogen Gas" in Circulation Research, Volume XIV, February, 1964. Since the detailed report of Aukland and co-workers, this method has been widely used to measure blood flow in diverse tissues including the brain. The method basically employs hydrogen-sensitive polargraphical electrodes of fine platinum wire that develop a current proportional to the partial pressure of hydrogen in surrounding tissue. When hydrogen administration is stopped and its concentration in arterial blood falls to zero, the clearance rate of hydrogen from the tissue is reflected as a proportional declining current from the electrode, is determined by local blood flow.
Hydrogen clearance possesses distinct advantages over other blood flow monitoring techniques. Hydrogen clearance can be determined in any tissue where small electrodes can be inserted. Second, multiple flow determinations can be obtained from the same tissue site over long periods of time unlike many other techniques. Third, blood flow can be estimated from the clearance rate of hydrogen independently of the absolute amplitude of the hydrogen signal. There is, however, some evidence that suggests that hydrogen clearance is not as accurate nor as local in measuring blood flow as generally supposed. Other problems stem from the failure to consider possible sources of error in hydrogen clearance monitoring and the invasive nature of the procedure. Nevertheless, hydrogen clearance is a valid and important approach in measuring blood flow.
Similarly, there is some criticism of the heat clearance method of determining local cerebral blood flow. The major criticism of the heat clearance method has been its lack of reliable quantitation.